[@robin-w Moderator 11 years, 7 months ago]

bboress just uses wordpress registration and login, so any plugin that works for wordpress will work for bbpress.

6 Useful WordPress CAPTCHA Plugins

bbpress also rides on askimet to help prevent spam entries, just check that you have enable it within the forum settings.

Unless your site has many new users, you could easily use manual registration, it takes less than a minute to set a user up in wordpress manually. That way you stop spammers dead in the water.